The present invention relates to a DC-DC converter, a power supply circuit, a method for controlling a DC-DC converter, and a method for controlling a power supply circuit.
Cellular phones have become compact. Power supply ICs used in power supply circuits of cellular phones thus have many outputs and are operated by low voltages. To prolong the life of batteries, the power consumption of the power supply circuits must be decreased.
FIG. 1 is a schematic circuit diagram of a prior art cellular phone power supply circuit 50. The power supply circuit 50 includes a DC-DC converter 1 and a low dropout (LDO) regulator 7.
In the DC-DC converter 1, a switching control circuit 2 controls the switching of a p-channel MOS transistor (first output transistor) Tr1. In the DC-DC converter 1, the switching of the first output transistor Tr1, an inductor 3, a capacitor 4, and a flyback diode 5 decrease a power supply voltage Vp to generate a first dropped voltage Vo1. A power supply terminal (not shown) of the LDO regulator 7 is supplied with the first output voltage Vo1 from a node N1 between the inductor 3 and the capacitor 4.
The LDO regulator 7 includes a p-channel MOS transistor (second output transistor) Tr2, which goes on in accordance with a comparison output signal of a comparator 8. In this state, the LDO regulator 7 decreases the first output voltage Vo1 to generate a second output (dropped) voltage Vo2. A load circuit (not shown) is supplied with the second output voltage Vo2.
The second output voltage Vo2 is divided by a first resistor R1 and a second resistor R2. The divided second output voltage Vo2 is provided to a plus input terminal of the comparator 8. An increase in the second output voltage Vo2 increases the comparison output signal of the comparator 8 and decreases an output (load) current Io2 of the second output transistor Tr2. A decrease in the second output voltage Vo2 decreases the comparison output signal of the comparator 8 and increases the output current Io2 of the second output transistor Tr2. Thus, the second output voltage Vo2 is maintained at a predetermined voltage, which is determined by the first and second resistors R1, R2.
The switching control circuit 2 and the comparator 8 receive a control signal CNT. The control signal CNT goes low when the load circuit, which receives the first and second output voltages Vo1, Vo2, enters a standby state. The low control signal CNT inactivates the switching control circuit 2 and the comparator 8 and causes the first and second output transistors Tr1, Tr2 to go off. This stops the supply of the first and second output voltages Vo1, Vo2.
The prior art portion of FIG. 6 illustrates the behavior of the second output voltage Vo2 when the power supply circuit 50 starts to receive power.
When a battery starts to supply power, the power supply voltage Vp supplied to the DC-DC converter 1 increases and the voltage of the control signal CNT increases synchronously with the power supply voltage Vp.
In the DC-DC converter 1, the operation of the switching control circuit 2 is stopped and the first output transistor Tr1 remains off until the power supply voltage Vp reaches a predetermined voltage V1. When the power supply voltage Vp increases to the predetermined voltage V1, the switching control circuit 2 starts to operate. This starts the switching of the first output transistor Tr1. In this state, the on time of the first output transistor Tr1 is controlled so that it gradually becomes longer to prevent problems caused by sudden increase of the first output voltage Vo1. Thus, the first output voltage Vo1 increases gradually. Subsequently, when the first output voltage Vo1 increases to a further predetermined voltage V2, the comparator 8 of the LDO regulator 7 starts to operate and increases the second output voltage.
In the power supply circuit 50, the switching control circuit 2 consumes consumption current I1 when the switching control circuit 2 starts to operate as the control signal CNT goes high, and the comparator 8 consumes consumption current I2 when the comparator 8 starts to operate.
The consumption currents I1, I2 are constant when the control signal CNT goes high and the first and second output voltages Vo1, Vo2 are output from the DC-DC converter 1 and the LDO regulator 7. Accordingly, as shown in the prior art portion of FIG. 6, the total current consumption (I1+I2) Icc of the switching control circuit 2 and the comparator 8 is constant regardless of the amount of the load current Io2 supplied to the load circuit by the LDO regulator 7.
Therefore, as the load current Io2 decreases, the ratio of the power consumed by the DC-DC converter 1 and the LDO regulator 7 increases. This decreases the efficiency of the power supply circuit 50.
Additionally, when the power supply circuit 50 starts to receive power, the first output voltage Vo1 increases gradually. This produces a relatively long delay time t1 from when the supply of power starts to when the second output voltage Vo2 increases to a predetermined level.
When the power supply circuit 50 is supplied with power, the first and second output transistors Tr1, Tr2 go off when the control signal CNT goes low. This decreases the first and second output voltages Vo1, Vo2. When the control signal CNT goes high again, the first output voltage Vo1 gradually increases. This produces a relatively long delay time t3 from when the control signal CNT goes high to when the second output voltage Vo2 increases to the predetermined level.
It is an object of the present invention to provide a DC-DC converter that decreases power consumption and quickly increases the output voltage.
To achieve the above object, the present invention provides a DC-DC converter including a voltage generation circuit for receiving a power supply voltage and generating an output voltage. The voltage generation circuit includes an output transistor for performing switching to generate the output voltage. A switching control circuit is connected to the voltage generation circuit to control the switching of the output transistor. A current detection circuit is connected to the voltage generation circuit to detect a load current derived from the output voltage of the voltage generation circuit to generate a detection signal. A stop control circuit is connected to the voltage generation circuit, the switching control circuit, and the current detection circuit to maintain the output transistor in an activated state in accordance with the detection signal and to stop the operation of the switching control circuit when the load current is less than or equal to a predetermined value.
A further perspective of the present invention is a power supply circuit including a DC-DC converter for receiving a power supply voltage and generating a first output voltage. The DC-DC converter includes a voltage generation circuit having a first output transistor. The first output transistor performs switching, and the voltage generating circuit generates the first output voltage in accordance with the switching of the first output transistor. A switching control circuit is connected to the voltage generation circuit to control the switching of the first output transistor. A voltage regulator is connected to the DC-DC converter to generate the second output voltage in accordance with the first output voltage. A current detection circuit detects a load current derived from the second output voltage of the voltage generation circuit to generate a first detection signal in accordance with the detected load current. A stop control circuit is connected to the voltage generation circuit, the switching control circuit, and the current detection circuit to maintain the output transistor in an activated state in accordance with the first detection signal and to stop the operation of the switching control circuit when the load current is less than or equal to a predetermined value.
A further perspective of the present invention is a method for controlling a DC-DC converter. The DC-DC converter includes an output transistor for performing switching to generate an output voltage and a switching control circuit connected to the voltage generation circuit for controlling the switching of the output transistor. The method includes supplying a load with a load current derived from the output voltage of the output transistor, detecting the load current, determining whether the load current is less than or equal to a predetermined value, maintaining the output transistor in an activated state when the load current is determined as being less than or equal to the predetermined value, and stopping the operation of the switching control circuit when the load current is determined as being less than or equal to the predetermined value.
A further perspective of the present invention is a method for controlling a power supply circuit. The power supply circuit includes a DC-DC converter having an output transistor for performing switching to generate a first output voltage and a switching control circuit for controlling the switching of the output transistor. A voltage regulator is connected to the DC-DC converter to generate a second output voltage in accordance with the first output voltage. The method includes supplying a load with a load current derived from the second output voltage, detecting the load current, determining whether the load current is less than or equal to a predetermined value, maintaining the output transistor in an activated state when the load current is determined as being less than or equal to the predetermined value, and stopping the operation of the switching control circuit when the load current is determined as being less than or equal to the predetermined value.
Other aspects and advantages of the present invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.